1. Field of the Invention
This invention relates to electrical assemblies and more particularly to solenoid and similar devices which have lead wires that connect internal components to an external junction outside the housing for the electrical device. Such electrical assemblies must have some means of preventing the ingress of moisture and other contaminants from migrating into the electrical device inside the housing.
2. Description of Related Art
Electrical devices, such as solenoid coils, will degrade and fail relatively quickly if the windings are exposed to moisture (rain, road salt, spray-down, submersion, etc.). These coils are often encapsulated in plastic for electrical isolation and this encapsulation affords the windings protection against direct water exposure as well. However, many solenoid coils have lead wires that run from the windings, through the plastic encapsulation, to the outside world creating an Indirect path for water ingress. This path exists because plastic encapsulants do not bond to lead wire insulation materials. Water (and aqueous solutions and mixtures) wicks into and moves along the interface between the lead wire insulation and the encapsulant to the windings, ultimately producing failure.
In U.S. Pat. No. 5,710,535, Goloff describes the use of elastomeric seals installed on each lead that are encapsulated along with the windings. The encapsulant, which is introduced around the coil assembly under significant pressure to form the housing, directly compresses the seal around each lead such that there is interference between the lead and the seal as well as between the encapsulant and the seal. However, a bond does not develop between the seal and the encapsulant and the dynamics of the molding process can distort the elastomer jeopardizing the soundness of the seal.
In U.S. Pat. No. 6,121,865, Dust et al. describe the use of an elastomeric seal that is installed around the leads after the coil has been encapsulated. In this method, the encapsulation mold is designed to produce a cavity around the leads where they exit the encapsulant forming the housing. The cavity formed in the encapsulant is sized to receive and compress the seal around the leads such that contaminants cannot penetrate the interface between the leads and the seal. The interface between the seal and the receiving cavity molded within the encapsulant is also under compression such that contaminants cannot penetrate this interface. However, some electrical assemblies, such as solenoids, cannot always accommodate pockets and seals where the leads exit the encapsulant forming the housing.
The Invention disclosed herein addresses the problem of contaminant ingress along leads in a practical way.
This invention prevents contaminants from migrating to the coil windings within an encapsulant forming the main housing through the use of a sealing assembly located within an over-molded, thermoplastic encapsulant. The seal assembly surrounds the insulated lead wires that extend from the coil windings either to outside the coil or to terminals that are molded into the free surface of the encapsulated coil. Before over-molding, an elastomeric seal is installed on each lead wire to be sealed, and this wire/seal subassembly is then inserted into a seal housing made from the same basic thermoplastic as the encapsulant forming the housing. The seal housing is constructed such that one or more continuous ribs, with sharp edges, circumvent the outer surface of the seal housing. During over-molding to form the main housing, the molten encapsulant surrounds the seal housing and melts the tips of the ribs. Upon cooling, the thermoplastic solidifies and the encapsulant bonds to the seal housing along each of its ribs.